Rewrite transpose_tiles to be in-place

[Rangi42]

Follow-up to #470. Makes the transpose_tiles function more efficient (no memory allocation, and no redundant assignment operations), with identical outputs.

for f in `find gfx -name '*.pic'`; do x=${f%.pic}; tools/pkmncompress -u $x.pic $x.tmp; cmp $x.tmp $x.2bpp; rm -f $x.tmp; done